VALLEY REGION ; ORIGIN OF THE ROOKS. 148 
direction of the thrust. In this way the upturned edges of 
the overridden side may be carried beyond the perpendicular 
and be actually reversed. Instances of this kind are com- 
mon enough ; the cross section given on another page shows 
it, particularly on the southeastern border of the Cahaba 
field, and on that of the Warrior field. 
In a similar way, when the break occurs near the bottom 
of a trough that has been shoved under an arch, the edges 
of tae under-shoved set will be bent or turned back more or 
less, and this also may go so far as to cause a reversal. We 
see this along the eastern edge of Murphree’s Valley almost 
its entire length. 
So far as I know, all the Alabama thrust faults have 
highly inclined or overturned strata on one side of the faults, 
and these vertical or reversed beds will be on the northwest 
or southeast side of the fault according to the character of 
the fault, whether a typical or a reversed one. Inthe great 
majority of cases the vertical or overturned strata are on 
the northwest side, for the reason that the great majority of 
the faults are typical ones. 
Usually the upturned edges occupy only a narrow belt, 
because part of them are generally below the surface, in the 
fault, and covered by the overriding measures ; but we have 
one magnificent example of the reversal of a great series of 
beds, in the overturned measures of the lower part of the 
Cahaba field, west of Montevallo, for here is a strip of the 
Coal Measures, two miles wide and six or seven miles long, 
pushed over beyond the perpendicular to an angle of 60°, 
and at the border of this strip we have the instance of the 
complete overturning of the measures and the gliding of the 
Cambrian strata over them, described in detail in another 
place and illustrated by a photographic view. 
The folds above spoken of are not symmetrical waves with 
crest and trough of equal width, but, as may be seen by any 
map of the Appalachian region, consist of rather narrow 
crests, with wide troughs between, in which the strata are 
either approximately horizontal or only slightly undulating. 
These troughs, or the most important ones, with raised 
edges and with the strata sloping from each'side towards 
the central line (synclinal), are the coal fields, which have to 
